1. Field of the Invention
The present invention relates to an air conditioner, and more particularly, to an apparatus and method for controlling super-heating degree, capable of preventing liquid compression of a compressor.
2. Description of the Related Art
The air conditioner is an apparatus for adjusting temperature, humidity, airflow, and cleanness of an air to achieve pleasant indoor environment. Recently, a multi-type air conditioner capable of arranging a plurality of indoor units for each installation space and adjusting air temperature for each installation space has been developed.
A heat pump system makes it possible to use a combined cooling system and heating system by using a cooling cycle principle for flowing a refrigerant through a normal channel and a heating cycle principle for flowing a refrigerant in reverse direction.
FIG. 1 shows a general cooling cycle and its relation on the Mollier chart. As shown in FIG. 1, in a cooling cycle, compression→liquidation→expansion→evaporation of a refrigerant are repeatedly performed.
A compressor 10 compresses an absorbed refrigerant and discharges a super-heated vapor of high temperature and high pressure, into an outdoor heat exchanger 15. At this time, the state of the refrigerant discharged from the compressor 10 is changed into a gas state of superheating degree beyond the saturated state on the Mollier chart.
The outdoor heat exchanger 15 generates a phase change of the refrigerant into a liquid state by exchanging heat from the refrigerant of high temperature and high pressure discharged from the compressor 10, with outdoor air. At this time, the refrigerant is rapidly lowered in its temperature by being deprived of its heat by air passing through the outdoor heat exchanger 15 and delivered as a liquid state of super-cooling degree.
Subsequently, an expansion apparatus 20 adjusts the refrigerant into a state where evaporation easily occurs in an indoor heat exchanger 25, by decompressing the refrigerant super-cooled at the outdoor heat exchanger 15.
In the meantime, an indoor heat exchanger 25 exchanges heat of the refrigerant that has been decompressed at the expansion apparatus 20, with heat of an outdoor air. At this time, the refrigerant is raised in its temperature by absorbing heat from an air passing through the indoor heat exchanger, whereby the phase of the refrigerant is changed into a gas state.
Also, the refrigerant absorbed to the compressor 10 from the indoor heat exchanger 25 becomes a gas state of super-heating degree (SH) that has evaporated beyond the saturated state.
From the above relation between the cooling cycle and the Mollier chart, it is understood that the refrigerant passes through the compressor 10, the outdoor heat exchanger 15, the expansion apparatus 20, the indoor heat exchanger 25, and goes back to the compressor 10.
Also, the refrigerant is changed in its phase into the state of the super-heating degree during the process that the refrigerant is delivered to the compressor 10 from the indoor heat exchanger 25. Namely, the refrigerant absorbed to the compressor 10 or discharged from the compressor 10 should be a complete gas state.
However, the foregoing is a theoretical result, and generally, an error occurs to some extent upon application of the system to an actual product. Furthermore, in case an amount of the refrigerant flowing on the cooling cycle is relatively large or small compared to the state heat-exchanged, the phase change at above each process is not complete.
Due to such a problem, the refrigerant absorbed from the indoor heat exchanger 25 to the compressor 10 may not be completely phase-changed into the super-heated vapor and still exit in the liquid state. When the refrigerant in the liquid state is accumulated in an accumulator (not shown) and then absorbed into the compressor 10, noise is increased and performance of the compressor is deteriorated.
Also, when a heating mode is switched into a defrosting mode or a defrosting mode is switched into a heating mode in the heat pump system, there is high possibility that the refrigerant in the liquid state is absorbed into the compressor 10. Such a phenomenon occurs as the refrigerant flow changes when the heat exchanger that has operated as the indoor heat exchanger operates as a condenser and, reversely, the heat exchanger that has operated as the outdoor heat exchanger operates as an evaporator during the mode switching process.
Also, the air conditioner according to the related art prevents the refrigerant in the liquid state from being excessively accumulated in the accumulator and being absorbed into the compressor, by adjusting the refrigerant flowing amount using the expansion apparatus 20 and getting the refrigerant absorbed to the compressor 10 to have a super-heating degree. Here, the expansion apparatus 20 includes LEV (Linear Electronic Expansion Value) or EEV (Electronic Expansion Valve), and is referred to as EEV hereinafter.
The air conditioner according to the related art, however, has the following problems.
When adjusting the refrigerant flow rate by controlling the expansion apparatus so that the difference between the discharging temperature of the compressor and the evaporating temperature of the outdoor heat exchanger may be maintained constant during the switching process between the heating mode and the defrosting mode, the liquid refrigerant may flow into the compressor, which is problematic.
Namely, for mode switching, switching by the 4-way valve is performed. At this time, if the compressor is operated simultaneously with mode switching, circulation direction of the refrigerant is reversed and the possibility that the liquid refrigerant is absorbed into the compressor gets increased.
Therefore, when the liquid refrigerant is absorbed into the compressor, there occurs a problem that the reliability of the product is lowered due to deterioration in performance of the compressor and noise generation.
Also, as the outdoor temperature is lowered, the difference between the temperature of the outdoor air and the temperature of the outdoor heat exchanger gets decreased, whereby heat exchange amount at the outdoor heat exchanger decreases and the liquid refrigerant amount accumulated in the accumulator increases and the possibility that the liquid refrigerant is absorbed into the compressor gets large. Such phenomenon acts as a factor that lowers reliability of the heat pump system.
Also, according to the related art, since response characteristics of the system depending on change of one degree in the absorbed temperature, gets very large, for control of the absorption super-heating degree, very accurate pressure sensor and temperature sensor are required.
Also, since the temperature computed on the basis of the high-saturated pressure is used for the reference for control of the discharging super-heating degree, the pressure at the lower pressure part and the refrigerant circulation amount are not considered, whereby an error increases, which is problematic.